1. Field of the Invention
This application relates to micro gas generators.
2. Description of the Prior Art
Micro gas generators are small pyrotechnic devices used to generate gas pressure to produce work. They are manufactured by a number of suppliers including LivBag (a subsidiary of AutoLiv), Special Devices, Inc., Davey Bickford, Nippon Kayaku, Takata Kogyo, OEA, and others.
In modern automobiles, seat belts are often fitted with pretensioners, designed to retract and tighten a seat belt around a passenger in the automobile, not merely to prevent its further extension. The pretensioner frequently uses a micro gas generator that is fired by a sensor mechanism indicating, for example, rapid deceleration of the automobile. The micro gas generator is contained in one end of a cylinder which also contains a piston. When the generator fires, the piston is driven down the cylinder and applies pressure to the seatbelt, retracting it and tightening is around the passenger. An animated cartoon showing a rotary seat belt pretensioner may be seen at http://www.autoliv.com/appl_alv/autoliv.nsf/pages/seat_belts_pretensioners. Micro gas generators are similarly used to inflate airbags for automobiles and for a number of other safety-related purposes.
It will readily be understood that a micro gas generator employed in an automobile, even though it may never fire during the life of the automobile, and is capable of firing only once, must be extremely reliable; and it is critical that the generator not be subject to such damage from environmental conditions and aging that it will not fire when properly initiated, and equally that it not be easily subject to improper initiation when not emplaced in its socket. For this reason, a hermetic seal is required around the charges and the ignition element; and a shorting device is required to prevent inadvertent initiation.
A typical micro gas generator, which is generally cylindrical, having a largest diameter of about 17 mm and a length of about 28 mm, is illustrated in cross-section in FIG. 1. In that Figure, two metallic contact pins 1,1xe2x80x2 pass through a polymeric header 2. The pins, which are sealed through the header 2, such as by buttress knurling of the pins and/or the use of an adhesive about the heads of the pins, carry current from an external source [for example, the control circuit that responds to rapid deceleration, when the micro gas generator is used in an automobile seat belt pretensioner] to a metallic bridge wire or similar ignition element 3 (such as a semiconductor bridgexe2x80x94see, for example, U.S. Pat. No. 5,912,427), which when electrically energized with an appropriate signal, produces a high temperature arc or spark to initiate the explosion of a primary charge 4 surrounding the bridge wire 3. Frequently, but not in the example of this Figure, the primary charge 4 is contained within a cup-shaped holder or inner can that attaches to the top of the header. The header 2 is fastened into the metallic base 5, typically by crimping, and sealed therewithin, typically by an O-ring 6. The assembly of the header and base and associated pins and bridge wire is crimped within a metallic output can 7 (sometimes referred to as a director can), which contains a secondary charge 8 to produce the necessary gas pressure output on ignition by the primary charge 4, and sealed by an O-ring 9. Typically, a shorting clip assembly 10, consisting of a polymeric holder and a shaped piece of spring metal held within the holder, not illustrated in detail in this Figure, is placed within the lower part of the base 5 below the header 2 so that the spring metal contacts both metallic pins 1, 1xe2x80x2 and shorts them when the generator is not mounted, preventing accidental ignition of the generator. The lower part of the base is typically fitted with sockets 11 to engage a clip from the wiring harness of the automobile, carrying trigger wires from the sensor circuit. The generator is placed into the seat belt pretensioner and the clip attached to the base; and attachment of the clip to the base separates the spring metal of the shorting clip assembly from the pins of the generator, allowing the generator to fire when the appropriate electrical signal is given by the sensor circuit.
The prior art design, although widely adopted, has a number of drawbacks:
(1) there are a large number of independent components, which must be manufactured and assembled.
These result in high production costs and the need for precise tolerances to avoid mismatching during assembly;
(2) there are four sealing paths: the two pins through the header, the header to the base (O-ring), and the base to the output can (O-ring). Each of the components around these seals must be precisely made and aligned to ensure a hermetic seal; and
(3) a separate shorting clip assembly must be manufactured and inserted into the micro gas generator to prevent accidental discharge. This requires the manufacture and assembly of additional components, adding cost and complexity.
During the manufacturing process, a test gas (such as krypton) is used to determine whether the micro gas generator has a hermetic seal. The generator is placed in a pressure chamber and the test gas allowed to fill the chamber under pressure and equilibrate. The test gas is then evacuated from the chamber and the generator is tested with a suitable detector, e.g. a gamma counter, to determine whether the test gas has entered and is leaking from the generator.
Examples of micro gas generators/igniters are given, for example, in U.S. Pat. Nos. 5,647,924; 5,648,634; 5,711,531; and 5,728,964.
There is thus an unmet need for a simpler and less expensive design for a micro gas generator, requiring fewer independent components.
The disclosures of the documents listed in this section and elsewhere throughout this application are incorporated herein by reference.
In a first aspect, this invention is a unitary header/base/shorting bar holder for a micro gas generator.
In a second aspect, this invention is a unitary header/base/shorting bar holder for a micro gas generator, further comprising a primary charge holder that is unitary with the header/base/shorting bar holder.
In a third aspect, this invention is a unitary header/base/shorting bar holder of the first or second aspects of the invention for a micro gas generator, further comprising a pair of contact pins sealed hermetically through the header.
In a fourth aspect, this invention is a unitary header/base/shorting bar holder of the third aspect of this invention, further comprising a shorting bar mounted within the shorting bar holder to short out the contact pins when the unitary header/base/shorting bar holder is not interlocked with a connector.
In a fifth aspect, this invention is a unitary header/base/shorting bar holder of any one of the first through fourth aspects of the invention for a micro gas generator, further comprising at least one vent to permit free escape of gas from the non-sealed volume of the generator into which the unitary header/base/shorting bar holder is assembled.
In a sixth aspect, this invention is a unitary header/base/shorting bar holder of any one of the first through fifth aspects of the invention for a micro gas generator, further comprising an interlocking feature in the base to interlock the unitary header/base/shorting bar holder with a connector.
In a seventh aspect, this invention is a unitary header/base/shorting bar holder of the sixth aspect of the invention for a micro gas generator, further comprising a keying feature to orient the unitary header/base/shorting bar holder with the connector.
In an eighth aspect, this invention is a micro gas generator incorporating the unitary header/base/shorting bar holder of any one of the first through seventh aspects of this invention. Such a micro gas generator consists essentially of:
(a) the unitary header/base/shorting bar holder of claim 1;
(b) a pair of parallel contact pins extending through the unitary header/base/shorting bar holder and sealed therewithin, so that each pin has a head protruding above the top of a header portion of the unitary header/base/shorting bar holder and a body protruding through a base portion thereof;
(c) a shorting bar, mounted on the base and shorting the bodies of the contact pins;
(d) an ignition element connecting the heads of the contact pins;
(e) a primary charge adjacent to the ignition element and adapted to be ignited thereby;
(f) optionally, a secondary charge to be ignited by the primary charge;
(g) a single O-ring, seated on an external shoulder of the base of the unitary header/base/shorting bar holder, the shoulder facing upwardly toward the header; and
(h) an output can, mounted over the unitary header/base/shorting bar holder from the direction of the header, sealingly engaged with the base thereof by compression of the O-ring, and crimped about a bottom edge of the base thereof.
In an ninth aspect, this invention is a method of assembling the micro gas generator of the eighth aspect of this invention.